. Earth Science News .
BLUE SKY
Geologists Revisit The Great Oxygenation Event

Banded iron formations, also known as taconite, are made of iron oxide layers alternating with iron-rich chert or jasper. The iron oxide is gray and the chert or jasper is generally red. One of the puzzles posed by these rocks is that the layers are only a few centimeters thick even though the formations themselves are massive.
by Staff Writers
St. Louis MO (SPX) Aug 26, 2010
In "The Sign of the Four" Sherlock Holmes tells Watson he has written a monograph on 140 forms of cigar-, cigarette-, and pipe-tobacco, "with colored plates illustrating the difference in the ash." He finds the ash invaluable for the identification of miscreants who happen to smoke during the commission of a crime.

But Sherlock Holmes and his cigarette ash and pipe dottle don't have a patch on geologists and the "redox proxies" from which they deduce chemical conditions early in Earth's history.

Redox proxies, such as the ratio of chromium isotopes in banded iron formations or the ratio of isotopes in sulfide particles trapped in diamonds, tell geologists indirectly whether the Earth' s atmosphere and oceans were reducing (inclined to give away electrons to other atoms) or oxidizing (inclined to glom onto them).

It makes all the difference: the bacterium that causes botulism, and the methanogens that make swamp gas are anaerobes, and thrive in reducing conditions. Badgers and butterflies, on the other hand, are aerobes, and require oxygen to keep going.

In the July issue of Nature Geoscience Washington University in St. Louis geochemist David Fike gives an unusually candid account of the difficulties his community faces in correctly interpreting redox proxies, issuing a call for denser sampling and more judicious reading of rock samples.

The world ocean
Fike, assistant professor of earth and planetary sciences in Arts and Sciences, focuses on the dramatic change from anoxic to oxygenated conditions in the world's oceans that preceded the Ediacaran period (from 635 to 542 million years ago) when the first multicellular animals appeared.

The primitive atmosphere of the Earth was probably made up of carbon dioxide, nitrogen and water, laced with methane, ammonia, sulfur dioxide, hydrogen sulfide and hydrochloric acid. What was missing? Oxygen.

If you look in a textbook, you'll find a story that goes something like this: Four billion years ago the earth's atmosphere was a deadly mixture of gases spewed forth by volcanoes: nitrogen and its oxides, carbon dioxide, methane, ammonia, sulfur dioxide and hydrogen sulfide.

The oceans that formed from condensing water vapor (or incoming comets) were reservoirs of dissolved iron pumped through hydrothermal vents on the ocean floor.

Then about 2.7 billion years ago, cyanobacteria, which have been called the most self-sufficient organisms on the planet because they can both photosynthesize and fix nitrogen, began bubbling oxygen into the atmosphere and shallow waters.

At first oxygen built up gradually in the atmosphere, but about 2.5 billion years ago there was a sudden spike upward, traditionally called the Great Oxygenation Event.

The oxygen killed off anaerobes that didn't find refuge in sediments, the deep ocean and other airless environments and led to the evolution of aerobes that could use oxygen to spark their metabolism.

At roughly the same time, iron began to precipitate out of the oceans, forming rocks peculiar to this period called banded iron formations that consist of alternating layers of gray and red rock.

Banded iron formations were created episodically from about 3 billion years ago until 1.8 billion years ago and almost never again.

The usual story is that iron was being swept from the oceans by increasing levels of dissolved oxygen.

And then, another two billion years after the Great Oxygenation Event, multicellular lifeforms finally put in an appearance. The first metazoans, as they are called, were the bizarre Ediacaran fauna, sometimes unflatteringly compared to sacks of mud and quilted mattresses.

The assumption was oxygen levels were now high enough to support something more than a single cell in lonely solitude.

Of course, this story has holes you could drive a truck through.

Why did oxygen levels spike 2.5 billion years ago, and how much oxygen was there in the atmosphere really? Why are banded iron formations made of layers only a few centimeters thick, and why did they stop forming so abruptly? If the oceans were oxygenated 2.5 billion years ago, why did multicellular life delay its appearance for another 2 billion years? And did all these changes really take place at pretty much the same time everywhere on Earth?

The problems arise, says Fike, because scientists don't have dense enough data to recognize spatial variations in Earth's geochemical past and because the geochemical proxies are so devilishly hard to interpret.

The world beach
The story started to fall apart in 1998, says Fike, when Don Canfield of Odense University in Denmark suggested that sulfur compounds had also played a role in the transformation of Earth's chemistry.

Canfield argued that the Great Oxygenation Event actually took place in two steps and that it was sulfides rather than oxygen that removed the iron from deep ocean water.

The first rise in oxygen caused oxidative weathering of rocks on land that delivered sulfates to the ocean through rivers and streams. In the ocean, sulfate-reducing bacteria converted the sulfates to sulfide to gain the energy they needed for daily housekeeping. The dissolved iron combined with the sulfides to form iron sulfide minerals such as pyrite that dropped out of solution.

During the second, much later stage, enough oxygen was generated to sweep the deep ocean of the toxic sulfides, ushering in the era of biological innovation, a.k.a. the mud sacks and quilted mattresses.

These transitions were still discussed as changes in bulk ocean chemistry - just from one anoxic chemistry to another anoxic chemistry.

However, in the July issue of Nature Geoscience, Simon Poulton of the University of Newcastle in England showed that sulfidic water protruded into the ocean only in a narrow wedge along the shorelines of ancient continents. This meant that the water column, instead of being homogeneous, was stratified, with different chemistries in different layers.

So much for the world ocean.

It's complicated
"Recent geochemical evidence indicates that, at least locally, ferruginous (iron rich) or even sulphidic (sulfur rich) conditions persisted through the Ediacaran period, long after the Great Oxygenation Event," Fike says.

"Things are much more complicated than we had supposed."

"As a community, we don't have a good sense of the spatial variation of these zones within different bodies of water," Fike says.

"What's more, different assessments can arise from the interpretation of different geochemical proxies, from physical separation between different ocean basins, or from the reworking of sediments after deposition," he continues.

The underlying problem is a low sampling rate. "As we try to unravel these changes in Earth's history, " Fike says, "we often don't have 100 different places where we can measure rocks of the same age. We're stuck with a few samples, and the natural tendency is to take your rocks and extrapolate."

The only way "to wring order from the chaos," Fike says, is to develop a full three-dimensional model of the Earth that has enough spatial resolution to wash out bad data.

A cautionary tale
At the same time, Fike acknowledges that spatial variability in redox proxies may make many geologists feel ill at ease because it might instead reflect an unusual depositional context or the reworking of the proxy after deposition instead of a signficant change in geochemistry.

By way of illustration, he describes a study of Amazonian mud belts, published this year by Robert Aller of Stony Brook University and colleagues in Geochimica et Cosmochimica Acta.

"The Amazon dumps mud rich in organic material into the Atlantic," Fike says. "The mud is deposited and the oxygen in it is consumed by biological activity, but then a storm churns it up, it gets reoxygenated, and redeposited. And this process happens over and over again."

By the time the mud becomes sediment, its chemistry is very different from what it was when it was first deposited.

"The redox indicators for the Amazonian sediments suggest that they were deposited under anoxic, sulfate-poor conditions, but we know they were deposited in well-oxygenated, sulfate-rich marine waters," Fike writes.

It is as if the murderer had deliberately removed cigar ash and substituted cigarette ash at the scene of the crime.

"Much work remains ahead of us before we can have a true sense of the three-dimensional redox structure of the oceans and how it varied through time," Fike concludes.



Share This Article With Planet Earth
del.icio.usdel.icio.us DiggDigg RedditReddit
YahooMyWebYahooMyWeb GoogleGoogle FacebookFacebook



Related Links
Washington University in St. Louis
The Air We Breathe at TerraDaily.com



Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News


BLUE SKY
NASA And Mavericks Launch Study Of Earth's Atmosphere
Moffett Field CA (SPX) Aug 17, 2010
NASA's Ames Research Center and Mavericks Civilian Space Foundation, Moffett Field, Calif., have announced a collaboration to develop a high-altitude high-velocity air sampling system for NASA biological experiments. Under the terms of a Space Act Agreement, Mavericks, in collaboration with NASA scientists, will develop and operate airborne science platforms to carry biological sampling de ... read more







BLUE SKY
Jazz breathes life back into New Orleans after Katrina

Chile seeks advice from NASA on feeding trapped miners

Chilean miners' rescue operation to last months

New Orleans police still pay for Katrina sins 5 years on

BLUE SKY
Apple expected to update iPod line at Sept. 1 event

Wired youth forget how to write in China and Japan

Toshiba to sell launch first 3D TV without glasses: report

US grants licenses for radar equipment sales to Taiwan

BLUE SKY
EU overfishing charges 'preposterous': Iceland

After decades, Estonians could regain seal hunting rights

Japan high-tech toilet makers flush with success

EU warns Iceland, Faroes over 'mackerel war'

BLUE SKY
Why Fish Don't Freeze In The Arctic Ocean

Receding ice could unlock arctic trove

Is The Ice In The Arctic Ocean Getting Thinner And Thinner

Resolving The Paradox Of The Antarctic Sea Ice

BLUE SKY
Plant Scientists Move Closer To Making Any Crop Drought-Tolerant

Ancient Roman mill uncovered in U.K.

Paraguay marks fragile farm-based recovery

Kenya fights threat to nut cash crop

BLUE SKY
Storm systems gather strength in the Atlantic

South Korea and China offer aid for North Korea flood crisis

Misery for Pakistan village cut off by flood

UN vows Pakistan relief despite threats

BLUE SKY
S.Africa defends Chinese expansion in Africa

S.Africa's Zuma in China for talks on growing ties

Somali peacekeepers may boost troops

South Africa's Zuma visits key partner China to boost ties

BLUE SKY
The Mother Of All Humans

Giant Chinese 'Michelin baby' startles doctors: reports

Mother Of All Humans Lived 200,000 Years Ago

Humans Trump Nature On Texas River


The content herein, unless otherwise known to be public domain, are Copyright 1995-2010 - SpaceDaily. AFP and UPI Wire Stories are copyright Agence France-Presse and United Press International. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by SpaceDaily on any Web page published or hosted by SpaceDaily. Privacy Statement